1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a bipolar transistor whose polysilicon layer serves as an emitter electrode and a semiconductor integrated circuit device including this type of bipolar transistor.
2. Description of the Related Art
Recently, it is the mainstream in a bipolar transistor to use a polysilicon layer formed on an emitter diffusion layer as an emitter leading electrode. This bipolar transistor is disclosed in, for example, IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. ED-33, NO. 11, NOVEMBER 1986, pp.1754 to 1768. "Physics, Technology, and Modeling of Polysilicon Emitter Contacts for VLSI Bipolar Transistors" GARY L. PATTON et al. The bipolar transistor has the structural advantage of, even if a junction of the emitter diffusion layer is shallow, increasing an effective emitter length (which corresponds to a distance between the emitter junction and the contact of metal wiring and polysilicon layer) by the polysilicon layer and thus preventing a base current from increasing.
In the bipolar transistor, however, recombination of minority carriers on the crystal grain interface of the polysilicon layer cannot be disregarded. The base current injected into the emitter diffusion layer is thus varied by the grain diameter of polysilicon, and a variation, which cannot be neglected, occurs in element characteristics such as a current amplification factor. Since the grain diameter of the polysilicon is largely influenced by a deposition temperature of the polysilicon layer, and also by a thermal treatment step included in the manufacturing method of the bipolar transistor, the uniformity of the polysilicon cannot be easily achieved.
The interface trap charge, which is the center of the recombination of the minority carriers on the crystal grain interface of the polysilicon on the emitter diffusion layer, originates from a dangling bond of a silicon atom existing on the grain interface.